A gas-diffusion electrode can comprise a semihydrophobic deposit of catalyzed high surface area carbon and fluorocarbon which is thermally sintered onto or into a planar substrate which may be carbon cloth, carbon paper or a metallic mesh or expanded metal. With respect to the principles of such electrodes, reference may be had to U.S. Pat. Nos. 4,614,575 and 5,047,133.
It is important with gas-diffusion electrodes to ensure a gas permeability of the body of the electrode which is such that electrolytes can enter the body only to a limited degree, to thereby avoid flooding of the pores of the body and any plenum, or other gas-diffusion system associated therewith.
By and large, one side of the body is supplied with gas while the other side faces the electrolyte, and it is common with such electrodes, especially when a free-flowing electrolyte is used and the electrode is immersed in the electrolyte to a considerable depth, to find it necessary to provide an antipercolation coating to prevent gas percolation losses to the electrolyte with a substantial reduction in electrochemical performance and gas economy.
In some cases, such percolation losses can be avoided by immobilizing the electrolyte by absorption into matrices or absorbent separators between electrodes.
Where the electrolyte, however, is capable of free flow, reacting gases can be prevented from migrating through the electrode by the capillary forces present where the electrolyte permeates the porous body. Where the balance between gas flow and electrolyte penetration is disrupted, the gas can percolate through the body without reacting or the electrolyte can pass through the body and flood the gas plenum or other distribution system and both the gas economy and electrochemical performance are adversely affected.
At small immersion depths, up to approximately 20 cm, the disruption by high reactant gas pressure or high electrolyte pressure is generally not a severe problem. However, as depth increases, the electrolyte tends to flood increasingly through the electrode to the gas plenum unless offset by gas pressure.
This danger can be combatted by compartmentalizing the electrode and controlling the isolated compartments so that they are of different gas pressures designed to resist penetration of the electrolyte through the porous body. This is costly and requires complicated engineering. Another method provides the gas-diffusion electrode with a microporous coating at the electrolyte side to allow fresh electrolyte wetting to carry the current, but restricts reactant gas percolation even at very high gas pressures. Such a coating allows construction of relatively low cost single compartment electrodes and allows more wide-spread use of gas-diffusion electrodes even in processes which have been able heretofore to use only solid electrodes.
There are two principal modes of use of gas-diffusion electrodes. In one mode, they are gasket-sealed into plate and frame electrolyzers which have a specific plenum for the supply of feed gas. In the second mode, they are laminated onto metal sheet substrates for use in immersed-tank electrolytic processes. In the latter case, the edges of the electrodes must be sealed to prevent gas leakage. In both cases, the antipercolation coatings are necessary where deep immersion in the electrolyte may be required.
Gas-diffusion electrodes with which the invention are primarily concerned are hydrogen-diffusion anodes, although the invention is also applicable to gas-diffusion cathodes. Hydrogen-diffusion anodes operate at electrode potentials which allow the use of a wide range of substrate and coating materials. Electrodes for oxygen reduction operate at much higher (oxidizing) potentials which can impose limitations on the materials used.
Improved immersed tank-type electrodes for application in the retrofitting of industrial electrolysis processes where the electrodes are required to operate at significant electrolyte depths have been developed. Such applications include replacing oxygen evolution as the anodic process in the electrowinning of metals such as zinc, copper and manganese. In such applications, hydrogen-diffusion electrodes must be typically immersed in an electrolyte in depths up to 6 feet and a traditional problem in such systems is that the feed gas can escape through the electrode structure into the electrolyte and/or the electrolyte will intrude through the electrode structure and flood the gas supply plenum. Experience has shown that, in many cases, only the top of the electrode, where the hydraulic pressure is the least, remains functional. Gas-diffusion electrodes of this latter type are described in U.S. Pat. No. 5,047,133.
In practice, full-scale industrial gas-diffusion anodes, laminated and sealed with conductive epoxy to metallic sheets, have been made in accordance with the teachings of this U.S. Pat. No. 5,047,133. Here the reactant gas is exclusively supplied over large electrode surface areas without any special plenum but by distribution directly through the carbon cloth.
In this earlier system, an anode percolation layer was coated onto the cloth which was bonded with conductive epoxy to a solid metal support. After the epoxy is applied to the substrate by screening to control thickness, the carbon cloth is applied and held in place by a vacuum bagging technique allowing a constant and uniform pressure to be applied during high-temperature curing of the epoxy.
Gas flow is through the cloth and between the epoxy (which intrudes into the cloth only to a limited extent) and the catalyst layer which also penetrates only to a limited extent into the cloth.
In the past, the antipercolation layers which have been used have not been fully satisfactory. In general, prior art antipercolation coatings for carbon-cloth diffusion layers of gas-diffusion electrodes were not entirely satisfactory.
As a practical matter, an antipercolation coating must have sufficient adhesion to catalyzed carbon cloth to overcome internal pressures which might tend to separate the antipercolation coating from the cloth and must provide acceptable gas retention, i.e. be free from pin holes. It must be stable in acid and/or base and in low concentrations of oxidizing agents and have good wettability with aqueous electrolytes and good ionic conductivity. The antipercolation coating also should show dimensional stability on hydration/dehydration and should form reliable bonds with conventional adhesives for sealing of edges.
It should be mechanically tough enough to resist punctures, capable of being patched preferably when wet with water or electrolytes, and should be of comparatively low cost, say no more than $50/m.sup.2.
Prior to the present invention, no antipercolation coating known to us has been able to satisfy all of these requirements.
For example, the antipercolation coating of U.S. Pat. No. 4,614,575 is a hydrogel which is applied to the catalyzed carbon cloth as a viscous solution which may be cross-linked into an immobile gel over a period of time.
The hydrogel was not fully satisfactory because it expanded excessively on hydration/dehydration and did not allow for successful edge-sealing with or without adhesives. Furthermore, fiber bundles could penetrate from the carbon cloth through the coating, which was applied in a liquid state, and these carbon fibers were capable of carrying sufficient hydrogen to cause unacceptable leakage of hydrogen during use.
The perfluorinated sulfonic acid membrane marketed under the trade name Nafion by DuPont has been proposed as an antipercolation layer for gas-diffusion electrodes but, at a cost of $700 per m.sup.2, it is too expensive for use in many applications. Bonded asbestos has also been used in the past as demonstrated in U.S. Pat. No. 4,435,267. Conventional adhesives do not bond well to either of these materials and thus they are unsuitable for immersed-tank applications where adhesive-bonded seals are required.